Output Coupling Capacitors

[johnsonad]

Having had the pleasure of listening to a couple of your rigs Steve, color me impressed!  Maybe it's time to get one myself

[ironbut]

Thanks for the compliments johnsonad.
IIRC you heard my Tape Project/electrostatic rig at the LA CanJam a little over a year ago.  Like I said at the time, the amp is in a constant state of flux and the next time you hear it, it should be totally different (in a good way I hope!).

[jrebman]

Steve,

Somewhat OT, but do you think you'll be coming to the RMAF CanJam in mid-October?

-- Jim

[johnsonad]

Steve I had a question for you, would you mind sending me an email please?

Regards,

Aaron

[ironbut]

Steve,

Somewhat OT, but do you think you'll be coming to the RMAF CanJam in mid-October?

-- Jim

It's penciled in on my calender. It always comes in on the heels of the Monterey Jazz Festival (a week or two after) so funds are always a question.

[jrebman]

Steve, sounds good.  Wish I could make it to Monterey too -- I have a cousin and her family who live there.  No money and travel is pretty difficult these days, but fortunately RMAF is just 40 minutes down the road from where I live.

-- Jim

[Laudanum]

The chart is for a voltage source/cap/load but the Crack has a 120 ohm output impedance. The cutoff in the bass goes with the total of 120 ohms plus the headphone impedance, so the 160-ohm line shows about what you can get with any low-impedance phone. You do not need a huge capacitor unless you do something (different tube, feedback, etc) to lower the output impedance.

Pauls comments should be self explanatory, but, Im dumb, so I need to ask a dumb question for clarity:

Does this mean that the Headphone impedance should be added to the 120 ohm amplifier output impedance and then this total impedance should be used to reference the chart for the -3db point?   So, with stock 100uf output cap,  for 32 ohm phones you would reference the chart for the -3db point of 152 ohms (120 ohm Crack output imped. + 32 ohm headphones), not 32 ohms ... so theoretical - 3db would be a little above 10 hz with 32 ohm phones, not 50 hz?

Or, am I once again missing "it".


Thanks.

[jrebman]

The load and resistors are in parallel, so they don't add as they do when they are in series.  In essence, to get the equivalent parallel resistance, you need to take the invere of the sum of reciprocal values, which essentially means that the total will always be less than the smallest resistance in the series.  Simple example would be two 10 ohm resistors: 1/10 + 1/10 = 2/10, inverted is 10/2, or 5.  This is the same basic formula for n number of resistors, just add up all the reciprocals and then invert that number to get the equivalent resistance.

From this you can also see that very high resistances in on the order of several k, will have very little effect on the lowest resistance in the series, so the 2.xk resistors across the headphone jacks are negligible.

Hope this helps,  I'm no teacher so I apologize for any assumptions I've made on your background.

-- Jim

[Laudanum]

Hi Jim

I should have stated my question differently.  I do know that parallel resistors dont add the resistors values together.  I used the wrong wording as I didnt mean "plus" in terms of resistors and the math of it.  I was wanting clarifiction on Pauls comments reqarding not needing a higher uF cap for lower -3db cutoff with low impedance phones.  

Paul stated ... "The chart is for a voltage source/cap/load but the Crack has a 120 ohm output impedance. The cutoff in the bass goes with the total of 120 ohms plus the headphone impedance, so the 160-ohm line shows about what you can get with any low-impedance phone. You do not need a huge capacitor unless you do something (different tube, feedback, etc) to lower the output impedance."

So what I was asking for was clarification on that.  My understanding of Pauls comments are that, the cutoff for most low impedance phones would be what is shown in the 160 ohm line in the chart, NOT the line corresponding to the actual stated impedance of the phones themselves.

 For a direct question .. would a 50 ohm headphone have a 32hz -3db cutoff  as directly shown by the 50 ohm line on the chart?  or would it have about a 10Hz -3db cutoff as corresponds to the 160 ohm line in that chart?  From Pauls comments, I was assuming the latter and that's what I was looking for clarification on as to whether I was  understanding that correctly or missing what he was saying.

Thanks

[jrebman]

Well, when it comes to charts, being blind as I am, I'm kind of useless at interpreting what I can't see :-).  Which is all to say that I can't answer your question on impedance.

Sorry for the unnecessary lesson in parallel resistance :-).

-- Jim

[Laudanum]

I appreciate the Reply.  Eventhough I did know about the parallel resistance I still added the output impedance to the headphone impedance to use as, what was, a very bad example and would logically elicit a lesson in parallel resistance like you gave.  So my fault all around for not thinking or explaining what I was getting at.  Im not very good at being succinct when typing and it was a brain fart on my part to boot.

Thanks for the effort.

[Paul Joppa]

Laudanum - in spite of the opioid implication :^) - has it correct. The source impedance (120 ohms resistive approximately) is in series with the load impedance (32 ohm headphones for instance) so the current loop consists of the output capacitor in series with the total impedance (152 ohms resistive in this case).

This analysis is based on a voltage source (zero impedance) in series with the source resistance, the output capacitance, and the load resistance.

[Laudanum]

Thank you once again, Paul.